/Engine/Plugins/Media/MediaIOFramework/Source/MediaIOCore/Public/MediaCapture.h
make sure the size of the SceneViewport doesn't change during capture.
True if the capture was successfully started
bool CaptureSceneViewport ( [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [FSceneViewport](API\Runtime\Engine\Slate\FSceneViewport) > & SceneViewport, [FMediaCaptureOptions](API\Plugins\MediaIOCore\FMediaCaptureOptions) CaptureOptions )